As is well known, a plural channel station carrier system provides for the simultaneous transmission of several conversations over the same transmission line between a central office and a plurality of remotely located subscriber telephone stations. Typically, the carrier equipment includes a central office transmitting and receiving channel terminal unit or circuit and a subscriber transmitting and receiving channel terminal unit or circuit for each of the several subscribers served by the carrier system. The central office channel terminal circuits are located at the telephone company's central office, and the subscriber channel terminal circuits are located remotely from the central office and are connected to the central office terminal circuits by the common transmission line. The subscriber channel terminal circuits form a part of what may collectively be referred to as the subscriber terminal equipment at the end of the transmission line remote from the central office.
To signal incoming calls for the subscribers served by the carrier system it is a common cost-saving practice to use just one ringing signal generator in the subscriber terminal equipment to serve all of the subscribers rather than using a separate local ringing generator for each subscriber.
When a call comes into the central office for one of the subscribers served by the carrier system, the central office-transmitted carrier signal assigned to the called subscriber will be modulated by an a.c. alerting signal. The local ringing signal generator at the subscribers' end of the transmission line responds to this modulation by applying an a.c. ringing signal voltage of relative low, fixed frequency (typically 20 Hz) to the called subscriber's telephone to operate the called subscriber's ringer.
The frequency of the carrier-modulating alerting signal mentioned above is typically selected to be much higher than the low ringing signal frequency that is customarily used for operating the telephone ringers. For example the frequency of the carrier-modulating alerting signal may be 750 Hz. The higher modulating signal frequency provides greater noise immunity and requires less expensive filters for separating the modulating signal from the other components of carrier signal detection in the carrier signal receiver equipment at the subscribers' end of the transmission line.
The foregoing type of station carrier system is sometimes installed in place of a party line (also referred to as a multiparty line) to furnish the party line subscribers with individual or single party service. The practice of equipping the carrier system with a single ringing generator to supply the same ringing signal for operating the ringers of all of the subscribers, however, leads to a problem where the party line to be converted uses a bridged frequency ringing system.
Bridged frequency ringing is a frequency selective ringing or signalling system wherein a different central office ringing signal frequency is assigned to each subscriber on the multiparty line and wherein the telephone ringer for each subscriber is tuned (usually capacitively) to the assigned ringing signal frequency so that it responds only to the assigned ringing signal frequency. In converting this type of system to a carrier operation, it has been common practice to use the same ringing generator for all of the subscribers and to replace the tuned ringers with ones capable of operation by the same ringing frequency. Replacement of the tuned ringers is nevertheless an added cost and requires a service man's visit to the home of each subscriber.
Another extra cost dealing with signalling arises where it is desired to use one of the carrier system's channels as a party line with bridged frequency ringing. To provide such a service one prior practice has involved the use of separate ringing signal generators which are located at the central office for supplying the different low ringing frequencies (e.g., 20 Hz, 30 Hz, 40 Hz) to which the subscribers' party line telephone ringers are tuned. In response to an incoming call for one of the party line subscribers the ringing signal frequency assigned to the called subscriber is applied to amplitude modulate a higher frequency tone (typically 900 Hz), and the modulated tone is then applied to amplitude modulate the carrier that is assigned to the party line channel.
At the subscriber terminal equipment this modulated carrier is received and demodulated to recover the modulated 900 Hz tone. The 900 Hz tone is then demodulated to recover the modulating ringing frequency information for operating the called subscriber's tuned ringer. Such a signalling system is shown in FIGS. 5 and 6 of U.S. Pat. No. 3,904,833 which issued on Sept. 9, 1975.
The cost of the signalling system shown in U.S. Pat. No. 3,904,833 is increased by the additional circuits needed for first modulating the 900 Hz tone at the central office terminal and then demodulating and filtering the modulated 900 Hz tone at the subscriber terminal to recover the desired ringing frequency information.
Instead of modulating the 900 Hz tone with the low frequency central office ringing signal (e.g., 20 Hz), some carrier systems are equipped to individually modulate the carrier signal with the low frequency central office ringing signal and the 900 Hz tone to thereby mix the low frequency ringing signal with the 900 Hz tone at the central office. Upon demodulation of the modulated carrier at the subscriber terminal equipment the low frequency ringing signal and the 900 Hz tone are recovered. The recovered 900 Hz tone is then used as switching signal to apply an amplification of the low frequency ringing signal to ring the called subscriber's telephone. This signalling system also requires additional circuits at extra expense to provide bridged frequency ringing for a party line on one of the carrier system's channels.
The present invention reduces the extra costs attendant with bridged frequency ringing as well as offering additional advantages as will become apparent from the following summary and detailed description.